1. Field of the Invention
The present invention relates to an exposure system for exposing photosensitive substrates, such as silicon plates and glass, to light through patterns designed for devices, such as semiconductors including an IC, an LSI, etc., a liquid crystal panel, a magnetic head, a CCD (image sensor), and so on.
2. Description of Related Art
In manufacturing an IC, an LSI, a liquid crystal element, etc., by photolithography, a projection aligner (projection exposure apparatus) is employed. The projection aligner is arranged to perform an exposure by projecting through a projection optical system a pattern of a photomask or a reticle (hereinafter referred to as a xe2x80x9cmaskxe2x80x9d) onto a substrate, such as a silicon plate or a glass plate, which is coated with a photoresist or the like (hereinafter referred to as a xe2x80x9cwaferxe2x80x9d in general).
FIG. 1 schematically illustrates the arrangement of a conventional projection aligner. In FIG. 1, there are illustrated a KrF excimer laser 251 used as a light source, an illumination optical system 252, illumination light 253, a mask 254, exposure light 255 on the object side, a projection optical system 256, exposure light 257 on the image side, a photosensitive substrate (wafer) 258, and a substrate stage 259 which holds the photosensitive substrate 258.
In the conventional projection aligner, a laser beam emitted from the excimer laser 251 is led to the illumination optical system 252. At the illumination optical system 252, the laser beam is converted into the illumination light 253 having a light intensity distribution, a luminous distribution, etc., which are predetermined. The illumination light 253 falls on the mask 254. A circuit pattern which is to be eventually formed on the photosensitive substrate 258 is beforehand formed on the mask 254 with chromium or the like. The incident illumination light 253 passes through the mask 254 and is diffracted by the circuit pattern to become the object-side exposure light 255. The projection optical system 256 converts the exposure light 255 into the image-side exposure light 257 to image the circuit pattern on the photosensitive substrate 258 at a predetermined magnification with sufficiently small aberrations. As shown in an enlarged view at the lower part of FIG. 1, the image-side exposure light 257 converges on the photosensitive substrate 258 at a predetermined NA (numerical aperture=sinxcex8) to form the image there. To have the circuit pattern formed in a plurality of shot areas on the photosensitive substrate 258, the substrate stage 259 is arranged to be movable stepwise to vary the relative positions of the photosensitive substrate 258 and the projection optical system 256.
However, with the conventional projection aligner using the KrF excimer laser arranged as described above, it is difficult to form a pattern image of a line width not greater than 0.15 xcexcm.
The reason for this difficulty is as follows. The resolution of the projection optical system is limited by a trade-off between an optical resolution and the depth of focus due to the wavelength of the exposure light. The resolution R of the resolving pattern of the projection aligner and the depth of focus DOF can be expressed by the following Rayleigh""s formulas (1) and (2):                     R        =                  k          ⁢                      xe2x80x83                    ⁢          1          ⁢                      λ            NA                                              (        1        )                                DOF        =                  k          ⁢                      xe2x80x83                    ⁢          2          ⁢                      λ                          NA              2                                                          (        2        )            
In the above formulas, xcex represents the wavelength of the exposure light, NA represents a numerical aperture indicative of the brightness of the optical system on the light exit side, and k1 and k2 represent constants which are normally between 0.5 and 0.7.
According to the formulas (1) and (2), in order to make the resolution R smaller for a higher degree of resolution, it is necessary either to make the wavelength xcex smaller for a shorter wavelength or to make the value NA larger for a higher degree of brightness. At the same time, however, the depth of focus DOF required for a necessary performance of the projection optical system must be kept at least at a certain value. This requirement imposes some limitation on the increase of the brightness value NA.
Meanwhile, known exposure apparatuses capable of giving also a high degree of resolution include probe-type exposure apparatuses.
FIG. 16 schematically shows the arrangement of a proximity-field probe exposure apparatus as one example of such probe-type exposure apparatuses.
Referring to FIG. 16, there are illustrated an exposure part 151, a laser light (beam) source 152, light source control means 153, an optical fiber transmission part 154, an optical fiber probe 155, an alignment part 156, a wafer 157, a wafer stage 158, and a wafer stage control part 159. In this example, the exposure part 151 which is arranged to generate exposure light is fixed. The wafer 157, which is a photosensitive substrate, is arranged to have its position controlled by moving the wafer stage 158 relative to the probe 155 according to information on the measured position of an alignment mark obtained with the alignment part 156 in its position (1). At the same time, the generation of exposure light from the probe 155 is controlled. Under such control, the wafer 157 is exposed to light of a circuit pattern in the neighborhood of its position (2).
The proximity-field probe exposure is carried out by introducing the exposure light into the optical fiber probe 155 which has its tip sharply formed. A circuit (exposure) pattern is formed on the wafer 157, i.e., the photosensitive substrate, by exposing the wafer 157 to a non-propagating component of the exposure light, i.e., proximity-field light, which seeps out from a microaperture part of the tip of the optical fiber probe 155, depends on the shape or size of the microaperture part and has a tiny spread less than a wavelength. FIG. 17 schematically shows an example of the optical fiber probe 155. In the case of this example, the probe 155 is completely covered with a metal coating 155b except an aperture part 155a of the tip for the purpose of efficiently converging the light propagating inside of the optical fiber toward the tip of the probe 155 and also for the purpose of preventing the S/N ratio of the exposure light from being degraded at the proximity field by scattering or transmission of non-proximity-field light taking place in the neighborhood of the aperture part 155a. 
The resolution of such an exposure system is determined by the aperture diameter and the sharpness of the tip of the optical fiber probe 155.
According to the current level of technology, an optical fiber probe can be prepared with the aperture diameter of the tip measuring less than 50 nm. The resolution of such a probe is much finer than that of the above-stated conventional projection aligner which has the aperture diameter of the tip of the probe at 200 nm or thereabout.
There are other known probe exposure methods, besides the above-stated method of using the proximity-field light. Other known probe exposure methods include a method called STM (scanning tunneling microscopy) which uses a tunneling current and a method called AFM (atomic force microscopy) which uses an interatomic force.
However, these probe exposure methods have a shortcoming in that the rate of throughput attainable by these methods is low. This is because an exposure area which can be covered by one shot of exposure in the probe exposure method is substantially the same as the minute size of the tip of the probe. In order to depict a circuit pattern over a wide exposure area, the wide area, therefore, must be exposed by spending much time.
FIG. 18 shows a probe part of the probe exposure apparatus, in which the probe part is made into a multiple probe so as to improve the rate of throughput by performing the simultaneous exposure using the multiple probe. For example, the rate of throughput can be increased by an N number of times by arranging the multiple probe to be composed of N probes 171 to 175. This improvement is furthered by a probe exposure apparatus having a much greater number of probes 181 which are two-dimensionally arranged as shown in FIG. 19. With the probe exposure apparatus arranged in this manner, a wide exposed area can be attained, like in the case of the conventional projection aligner, by carrying out a block (or batch) exposure. Then, the wafer can be exposed at an improved rate of throughput. Further, the resolution attainable by such apparatus can be improved by obliquely arranging the probes with respect to the two-dimensional array.
FIG. 20 shows by way example the arrangement of making a probe of a type which differs from the optical fiber probe into a multiple probe. In the case of FIG. 20, a surface emitting laser 190, a microaperture 197 and a cantilever 199 are used for each unit of the multiple probe.
Although a high degree of resolution having a line width not exceeding 100 nm can be attained by the probe exposure described above, it is hardly possible to form circuit patterns at a high rate of throughput required by an actual manufacturing operation.
It is an object of the invention to provide an exposure system which gives a high throughput with a high degree of resolution.
To attain the above object, in accordance with an aspect of the invention, there is provided an exposure system, which comprises a first exposure part arranged to perform a probe exposure, and a second exposure part arranged to perform a projection exposure, wherein the exposure system has a multiple exposure mode in which a multiple exposure is performed by using both the first exposure part and the second exposure part in combination.
Further, the probe exposure in the multiple exposure mode is performed in such a manner that a first exposure pattern having an exposure amount not exceeding a threshold value of an object (resist) to be exposed is formed, the projection exposure in the multiple exposure mode is performed in such a manner that a second exposure pattern having an exposure amount exceeding the threshold value and an exposure amount not exceeding the threshold value is formed, and the respective exposure amounts are determined in such a manner that a composite exposure pattern formed by combining the first and second exposure patterns is in such a relation to the threshold value that a desired circuit pattern is formed.